1. Field of the Invention
The present invention relates to low dielectric constant substrates comprising cordierite and methods for forming the low dielectric constant substrates.
The low dielectric constant substrates of the present invention find particular application in forming multilayer ceramic materials.
The substrates may be designed with termination pads for attaching semiconductor chips, connector leads, capacitors, resistors, covers, etc. Three dimensional current networks may be produced using the so-called MLC or multilayer ceramic process in which horizontal circuit patterns that are thick film screened on unsintered or green sheets are connected by using vertical interconnections or vias formed by metal paste filled holes. Individual layers are then laminated and then sintered forming a sintered, dense ceramic/metal package.
2. Description of the Prior Art
A relatively recent innovation in electronic packaging has been the development of the multilayer ceramic (hereafter often MLC) module. In this technology, "green" sheets of ceramic powder held together by a temporary organic binder are metallized with a noble or refractory metal, usually, but not mandatorily, by screen printing. The metallized sheets are stacked, laminated and fired to form a monolithic ceramic-metal package. Details on MLC technology are given in SOLID STATE TECHNOLOGY, May, 1972, Vol. 15, No. 5, pages 35-40, Kaiser et al, hereby incorporated by reference, and U.S. Pat. No. 2,966,719, also hereby incorporated by reference.
Additional procedures for forming laminated green sheets for fabricating multilayer substrates are also disclosed in U.S. Pat. Nos. 3,423,517 and 3,723,176, both hereby incorporated by reference.
IBM Technical Disclosure Bulletin, Vol. 16, No. 4, September, 1973, page 1282, discloses a method of depositing thick gold lines over interconnection metallurgy located on a ceramic substrate. Molybdenum interconnection patterns and via paths are disclosed.
IBM Technical Disclosure Bulletin, Vol. 19, No. 4, September, 1976, page 1259, discloses a plated ceramic module. A layer of ceramic is coated with fritted molybdenum which in turn is coated with a layer of molybdenum to insure a continuous plating of nickel that can be diffused into the entire molybdenum layer.
IBM Technical Disclosure Bulletin, Vol. 19, No. 10, March, 1977, page 3777, discloses the use of high-density sintered molybdenum for internal circuit patterns and for the top and bottom side metallurgy of multilayer ceramic modules. Since the molybdenum is subject to corrosion problems, it is protected during the sintering operation by forming a corrosion-resistant nickel-molybdenum layer thereon.
U.S. Pat. No. 2,731,355 to Skinner discloses magnesium-aluminum-silicate compositions and methods of preparing the same. More particularly, Skinner relates to compositions of this type having high crystalline cordierite development and to methods of producing the same. The compositions are stated to have good electrical properties and low thermal expansion.
U.S. Pat. No. 2,864,919 to Stringfellow discloses ceramic compositions useful as arc extinguishing means in connection with circuit interrupting equipment. The compositions comprise a substantial quantity of cordierite crystals, specifically more than 50% of such crystals, having a specific ultimate composition which can be defined on an MgO-SiO.sub.2 -Al.sub.2 O.sub.3 phase diagram as 5 to 15% MgO, 22 to 58% Al.sub.2 O.sub.3 and 38 to 63% SiO.sub.2.
U.S. Pat. No. 3,040,213 to Byer et al. discloses composite "glaceramic" bodies useful in the fabrication of electrical components such as printed circuits which can be integrally united or interfused to produce a monolithic structure from separate preformed bodies.
U.S. Pat. No. 3,246,972 to Smith et al. discloses that cordierite has been commonly resorted to for the formation of compositions having high thermal shock resistance, high temperature and chemical durability and low thermal expansion but that it suffers from the problem that its expansion coefficient is high enough to limit the thermal shock that articles made therefrom can withstand and to cause excessive warpage of such articles.
U.S. Pat. No. 3,824,196 to Benbow et al. discloses that cordierite has been utilized in catalyst supports.
U.S. Pat. No. 3,885,977 to Lachman discloses an extruded, honeycombed monolithic fired ceramic whose primary crystal phase is cordierite and whose microstructure is characterized by a greater than random orientation of anisotropic cordierite crystallites. The product is particularly adapted for use as a catalytic support matrix for emission control.
U.S. Pat. No. 3,954,672 to Somers et al. discloses cordierite refractory compositions suitable for making a catalyst substrate as well as other ceramic bodies. Somers et al. discloses the firing of green, dried monoliths in air at a temperature of at least 2,550.degree. F. to enable the formation of the maximum amount of cordierite phase.
U.S. Pat. No. 3,993,821 to Goss discloses the use of molybdenum as a metallization for application to green, unfired beryllia ceramics in single or multilayer laminates.
U.S. Pat. No. 4,109,377 to Blazick et al. discloses a method for manufacturing a multilayer ceramic which is particularly suitable for carrying semiconductor chips wherein a particulate mixture containing a metal and the metal's oxide in a ratio of between 1:1 to 9:1 is deposited in a pattern on at least a portion of a plurality of ceramic layers, the patterns are then dried and laminated under substantial pressure and fired at an elevated temperature. The metal oxide allows shrinkage of the metallization so that it more nearly matches that of the ceramic. Preferred metals and metal oxides include molybdenum powder and molybdenum trioxide powder.
U.S. Pat. No. 4,153,491 to Swiss discloses a green ceramic sheet adaptable for accelerated sintering comprising a high alumina ceramic green sheet having an average particle size greater than 1 micron.
U.S. Pat. No. 4,234,367 to Herron et al. discloses the formation of sintered glass-ceramic substrates containing multilevel interconnected thick-film circuit patterns of copper-based conductors obtained by firing in a controlled ambient of hydrogen and H.sub.2 O at temperatures below the melting point of copper.
U.S. Pat. No. 4,301,324 to Kumar et al. discloses sintered glass-ceramic substrates containing multilevel, interconnected thick-film circuit patterns of highly conductive metals which can be fired in air or in neutral atmospheres at temperatures below the melting point of the metals. The invention is based upon the discovery that finely divided powders of certain glasses sinter to essentially zero porosity at temperatures below 1,000.degree. C.
Japanese Patent Application No. 53-71269 discloses a multilayer circuit substrate produced by preparing an unfired multilayer ceramic substrate comprising conductor circuit layers each having, as an electroconductive material, a high melting metal powder which can be molybdenum plated with a noble metal.
Japanese Patent Application No. 55-42516 discloses a ceramic circuit board including buried conductor layers and surface conductor layers where the surface conductor layers can be made of molybdenum and include a copper or nickel-copper plated layer.